It is well known in the field of film production that the modulus or stiffness of a resin film increases with increasing resin density. Correspondingly, lower resin density usually results in increased impact and tear strengths. In the case of films of resins made by conventional Ziegler catalysts (which are typical examples of non-single-site polymerization catalysts) alone, improving the stiffness of a film construction, a requirement for many industrial and packaging films, entails inevitable tradeoffs, i.e., increased density to improve stiffness typically sacrifices impact and tear strengths.
Lower haze and higher gloss are other film attributes that improve with lower resin density. If both haze and gloss as well as stiffness of a film are to be optimized, another compromise has to be made with conventional Ziegler catalyzed resins. Therefore, it would be desirable to have available a resin, in particular, a polyethylene resin which despite a relatively low density and favorable film properties associated therewith such as, e.g., high impact and tear strengths (toughness) as well as low haze and high gloss, shows a modulus (stiffness) that renders the films made therefrom suitable for applications such as industrial and packaging film.
U.S. Pat. No. 6,051,525 to Lo et al., the disclosure of which is expressly incorporated herein by reference in its entirety, discloses a catalyst composition for preparing a high activity catalyst supported on silica which produces, in a single reactor, polyethylene with a broad or bimodal molecular weight distribution. The catalyst is prepared from the interaction of calcined silica with dibutylmagnesium, 1-butanol and titanium tetrachloride and a solution of methylalumoxane and ethylenebis[1-indenyl]zirconium dichloride.
U.S. Pat. No. 5,539,076 to Nowlin et al., the disclosure of which is expressly incorporated herein by reference in its entirety, discloses resins which are in situ catalytically produced polyethylene resin blends of a broad bimodal MWD that can be processed into films on existing equipment and exhibit good processability in blown film production, reduced tendency towards die-lip buildup and smoking in on-line operations. The preferred catalyst for producing these resins comprises a catalyst including a support treated with a dialkylmagnesium compound, an aluminoxane, at least one metallocene and a non-metallocene transition metal source as well as an alkylaluminum compound as cocatalyst.
U.S. Pat. No. 5,614,456 to Mink et al., the disclosure of which is expressly incorporated herein by reference in its entirety, is directed to an activated catalyst composition for producing bimodal MWD high density and linear low density polyethylene resins, which activated catalyst does not require alkylaluminum cocatalyst. A preferred catalyst comprises, as support, silica impregnated with a dialkylmagnesium compound and an organic alcohol reagent, e.g., butanol. Said support is contacted with at least two transition metal compounds, at least one of which is a metallocene, and, as activator, aluminoxane, either alone or admixed with metallocene compound.
Other background references include EP 0 286 177, EP 0 643 084, Kyung-Jun Chu et al., “Variation of molecular weight distribution (MWD) and short chain branching distribution (SCBD) of ethylene/1-hexene copolymers produced with different in-situ supported metallocene catalysts,” Macromol. Chem. Phys. 201 (2000), 340-348, and literature cited therein, C. Gabriel et al., “Comparison of different methods for the investigation of the short-chain branching distribution of LLDPE,” Polymer 42 (2001) 297-303, and literature cited therein, and J. D. Kim et al., “Copolymerization of Ethylene and 1-Hexene with Supported Metallocene Catalysts: Effect of Support Treatment, Macromol. Rapid Commun. 20 (1999), 347-350, and the literature cited therein.